(1) Field of the Invention
The invention relates in general to a method and an arrangement for transferring packet data in the radio access network of a cellular system. The invention relates in particular to a situation where many mobile stations share a packet data channel.
(2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98;
Traditionally cellular systems, for example the Global System for Mobile telecommunications (GSM), have been used to transmit speech and they have implemented circuit switching. In circuit switching a certain amount of transmission resources is reserved in all the networks through which the connection goes. For new data applications there is usually need to transmit bursts of data every now and then. For this kind of data transmission circuit switching is not an efficient way to transmit data.
General Packet Radio Service (GPRS) is an example of a wireless packet switched network. It is an addition to the GSM system. Using GPRS it is possible provide a certain portion of the radio resources of the GSM radio access network for users who wish to transmit packet data. The statistical multiplexing, i.e. the fact that every user is not transmitting packets at the same time, allows a certain radio channel to be used efficiently by many users. Similarly, Enhanced Packet Radio Service (EGPRS) is a packet data system of the EDGE (Enhanced Data Rate for GSM Evolution). EDGE is an enhanced version of GSM that provides circuit switched and packet switched data transmission at a higher rate than current GSM or GPRS. EDGE and EGPRS are under specification at the priority date of this patent application.
The packet data channel, as most channels in Time Division Multiple Access (TDMA) systems, consists of a sequence of radio bursts which are sent in a certain, predetermined time slot in sequential frames. In circuit switched connections each time slot is reserved for a certain mobile station during the whole connection, so there can be, for example, some specific signaling which takes place each time a circuit-switched connection is set up or torn down. In the downlink direction the network may transmit information to various mobile stations using one packet data channel. In the uplink direction, the packet data channel is a channel shared by many mobile stations, so there has to be a method to control the uplink transmissions of the mobile stations on a packet data channel.
FIG. 1 presents a schematic diagram of a GSM radio access network (RAN) 110, which can transmit GPRS data, and GPRS core network 120 as an example of a wireless packet radio. A mobile station (MS) 101 communicates with a base station (BTS) 102a. One or more base stations, in FIG. 1 base stations 102a and 102b, are connected to a base station controller (BSC) 103. The base station controller is responsible, for example, for allocation of radio resources and for handling handovers, where a mobile station changes the base station it communicates with. The base stations and base station controllers form the GSM radio access network 110. In addition to these components, a GSM network comprises in the fixed part of the network Mobile Service Switching centers (MSC), Home Location Register (HLR) and Visitor Location Register (VLR). HLR and VLR take part in, for example, subscriber and mobility management. These are not shown in FIG. 1.
The GPRS core network comprises GPRS supporting nodes (GSN). Of these nodes, the one which is on the edge towards a data network 130, for example the Internet, is called Gateway GPRS supporting node (GGSN). In FIG. 1, a GGSN 105 is presented. Data packets may run through many GSNs, which act as routers. A mobile station, which is the endpoint of the data connection, is reachable through one base station controller and the GSN connected to this base station controller is called Serving GPRS support node (SGSN). In FIG. 1, the mobile station 101 is reachable via the BSC 103 and the GSN connected to this BSC is SGSN 104.
User data is transferred transparently between the MS and the external data networks with a method known as encapsulation and tunneling: data packets are equipped with GPRS-specific protocol information and transferred between the MS and GGSN. In order to access the GPRS services, a MS first makes its presence known to the network by performing a GPRS attach. This operation establishes a logical link between the MS and the SGSN, and makes the MS available for, for example, paging via SGSN and notification of incoming GPRS data.
The SGSN is at the same hierarchical level as the MSC, keeps track of the individual MSs' location and performs security functions and access control. The Gateway GSN provides interworking with external packet-switched networks, and is connected with SGSNs via an IP-based GPRS backbone network.
Functions applying digital data transmission protocols are usually described as a stack according to the OSI (Open Systems Interface) model, where the tasks of the various layers of the stack, as well as data transmission between the layers, are exactly defined. FIG. 2 presents the lowest protocol layers of the GSM RAN and the lowest protocol layers of a mobile station MS. The lowest protocol layer between the mobile station MS and the base station subsystem is layer 1 (L1) 200, 201, which corresponds to a physical radio connection. Above it, there is an entity 210, 211 corresponding to the layers 2 and 3 (L2/3) of a regular OSI model. The lowest sublayer in this entity is a radio link control/media access control (RLC/MAC) layer 202, 203. On top of it there is a logical link control (LLC) layer 204, 205 and the topmost sublayer in the entity 200, 201 is a radio resource control (RRC) layer 206, 207.
The mobile station MS includes also a higher-level control protocol 212, which communicates with the RRC layer 206 in order to realize control functions connected to data transmission connections. It also includes a protocol 213 for serving higher-level applications, which communicates directly with the LLC layer 204 in order to transmit such data that directly serves the user (for instance digitally encoded speech). In the mobile station of the GSM system, blocks 212 and 213 are included in a Mobility Management layer.
In GPRS, a Temporary Block Flow (TBF) is created for transferring data packets on a packet data channel. The TBF is a physical connection used by two mutually communicating Radio Resource (RR) peer entities to support the unidirectional transfer of Logical Link Control (LLC) Packet Data Units (PDU) from upper Logical Link Control (LLC) layers on physical channels for packet data. There are separate TBFs for the uplink and downlink directions, even if the connection set up by a higher protocol layer is bidirectional. Usually there is only one TBF uplink-downlink pair per mobile station.
During an uplink TBF the mobile station will organize the data to be transferred into Protocol Data Units or PDUs. These are in turn divided into smaller parts which are distributed into data blocks on the RLC layer which defines the procedures related to information transfer over the radio interface. Each RLC block will have a certain connection identifier, for example the Temporary Block Flow Identifier (TFI), corresponding to the TBF whose data it carries. During a downlink TBF a similar arrangement of successive RLC blocks is produced by the network and transmitted to the mobile station.
When the RLC blocks are transmitted over the radio interface, a MAC header is attached to the coded RLC block. Here term coding refers to channel coding. The transmitted coded data blocks are called RLC/MAC blocks. The coding adds redundancy to the data, and the aim of coding is to recover the data even if some occasional transmission errors occurs. In addition to coding, the data is usually also interleaved. This means, for example, that sequential data chunks are not sent one after other, but in some other order. This way more bursty transmission errors can be tolerated.
FIG. 3 illustrates schematically a packet data channel in GSM. In FIG. 3 there are eight sequential TDMA timeframes having reference numbers starting with number 300. Each timeframe consists of eight time slots; in FIG. 3 these have reference numbers starting with numbers 301–308. A channel in GSM is specified by the time slot number within a certain timeframe sequence. The timeframes of a certain sequence (i.e. related to a certain channel) can be transmitted using various frequencies.
In FIG. 3 presents a channel which uses the time slots having reference numbers starting with number 303. The data transmitted in a time slot is carried on a radio burst that lasts the duration of the time slot. In packet data channels, a number of sequential bursts in a certain time slot forms a radio block. The number of radio bursts per radio block is four in the current EGPRS specifications, where one radio block carries one coded RLC/MAC block. FIG. 3 illustrates two sequential radio blocks of a certain packet data channel. It is also possible that a packet data channel uses also other time slots than just one certain time slot in each frame.
FIG. 4 shows schematically a coded and interleaved downlink RLC/MAC block 400 that is transmitted using four radio bursts. Only the relevant fields of the RLC/MAC block are presented, and the data carried by each radio burst is presented with a horizontal rectangles (rectangles 401, 402, 403 and 404). The MAC header is coded in a coding scheme that depends on the modulation that is used. The modulation may directly dictate the coding scheme of the MAC header. If there are several possible coding schemes for a certain modulation, then the coding scheme that is used must usually be somehow specified in the coded RLC/MAC block. This can be done, for example, by inserting or adding a certain bit string to the coded MAC header. The bit string is usually called Stealing Bits (SB).
The contents of the RLC block do not affect the coding of the MAC header. The coded MAC header is interleaved on four radio bursts. This is presented in FIG. 4 by placing the (coded) MAC header 411 vertically. The MAC header comprises an uplink state flag (USF) field which declares the mobile station who is allowed to use the next radio block in the uplink packet data channel. The length of the USF field is 3 bits in GPRS and EGPRS, so up to eight mobile stations can share an uplink packet data channel. The USF field 412 of the header is coded in a way that the rest of the data in the header does not affect. The coded USF is presented separately inside the MAC header 411.
The RLC block is coded using a certain coding scheme and the coded data is interleaved on that part of the radio bursts which is left over from the coded MAC header. The coded and interleaved RLC block is presented in FIG. 4 in four chunks 413, 414, 415, 416. Each of the data chunks is carried by one radio bursts.
For a mobile station to know if the data carried by a downlink RLC/MAC block is for itself, it has to first receive information carried by a radio block and then uncode the information. The coding scheme of the RLC block has to be known to the mobile station, and it can be, for example, defined when a TBF is activated. Alternatively, it can be indicated with a certain bit string in the MAC header. Once the RLC block is uncoded, the mobile station can determine the connection identifier, for example the TFI, in the RLC block. The TFI indicates to which mobile station the data is sent.
The current way of transmitting a coded RLC/MAC data block uses one radio block. A radio block allocated to a mobile station when it either transmits or receives packet data. In other words, a packet data channel can be shared between the mobile stations (or users) with block-by-block multiplexing. Term block here refers here to a radio block.
Certain real time applications, such as speech, pose very strict delay requirements. This dictates that data for these applications must be transmitted, for example, in every radio block of a packet data channel. The amount of data that the application produces, however, may be less than the amount of data that can be carried in a packet data channel reserved totally for this mobile station. It is possible to transmit the speech data in every other radio block of a certain packet data channel, but this may increase the delay too much. The current block-by-block multiplexing on a packet data channel may thus lead to inefficient use of the radio resources.
A solution to this problem has been proposed in U.S. patent application 60/144,307. A new radio burst for speech that is transmitted in EDGE using 8-PSK modulation is defined there. The suggested new radio burst lasts, for example, half the time of an ordinary radio burst. This way it is possible to fit two speech connections into one timeslot. The suggested scheme works only for 8-PSK modulation.
A further problem is that to fulfill certain quality of service constrains a certain application may have, it may be necessary to change the coding and/or interleaving properties during a packet data connection. The quality of service requirements of the application, for example, affect the parameters of the TBF. Finnish patent application FI991382 discusses in detail how and where the quality of service requirements of a packet data connection are translated to parameters of the TBF. The translation takes place most probably in and between the protocol layers 2 and 3. The patent application FI991382 does not discuss how to support the various TBFs over the radio interface on the protocol layer 1.
Finnish patent application FI990538 proposes the use of multiple TBFs per unidirectional radio connection between a mobile station and a radio access network. Each of the TBFs corresponds to different quality of service parameters. By activating a new TBF before closing the old one prevents a break in the packet data connection due to the activation of a new TBF. This method requires that it is possible to predict the quality of service requirements of a certain packet data connection.
The various quality of service requirements discussed, for example, in both above mentioned Finnish patent applications imply the use various coding and interleaving schemes. Further, different coding and interleaving schemes generally produce various amounts of coded data that needs to be transmitted per one RLC/MAC block. The amount of coded data to be transmitted is most probably not always a multiple of the data that can be transmitted using a radio block. The current way of allocating radio resources in radio blocks may thus in many situations lead to inefficient use of radio resources.
There has been a proposal on burst-by-burst multiplexing in the uplink direction. In this proposal, the number of USF fields in the MAC header is increased. Introducing more than one USF fields to the MAC header causes compatibility problems. A mobile station that is designed to look for one USF field may find the first of the USF fields in a MAC header. It cannot, however, detect the rest of the USF fields and therefore it may not notice that it is its turn to use the uplink packet data channel. The change of size of the MAC header also causes changes to the coding and inter-leaving of the downlink RLC blocks. Therefore, using this proposal, the mobile stations capable of understanding multiple USF fields in the MAC header must be multiplexed on a different packet data channel than the rest of the mobile stations.
The object of the invention is to provide a flexible method for transmitting data on a packet data channel. The data may be related to various packet data connections, mobile stations or users. The object of the invention is also to present a method for designating data transmission capacity to the mobile stations in variable sized chunks. A further object is to provide a method that can be used in uplink and/or downlink direction. The method is preferably compatible with earlier methods for multiplexing mobile stations or packet data connections on packet data channels.
The objects of the invention are achieved by selecting from set of values the number of radio bursts using which a data block is transmitted.